How To Prove It

Starting with this issue we will run a regular column on the art and science
of proof, and in honour of George Pólya’s book, ‘How To Solve It’, we have
named it "How To Prove It." There is of course no single way to prove things in
mathematics. But there are many general ideas and strategies that do help, and
that’s what this column is about.
Formal proof is one of the striking features of mathematics. You
do not find this feature in any of the sciences. What you do meet in
the sciences would be more accurately described as ‘verification’.
You may for example perform an experiment in the laboratory to
verify the formula t =
for the time period of oscillation of
a pendulum. What do you do? You set up the apparatus and take a
lot of readings, then draw a graph or two and check how close are
your results to the prediction. At the end you say, ‘The formula has
been verified to be true within experimental error’ or something
like that. This is done routinely in the sciences. It is important to see
that this is not the same as proof in mathematics.
in the classroom
How To Prove It
S������� A S������
In a proof what you are attempting to do is to build a logical bridge
from one set of statements (or suppositions) to another statement,
using intermediate steps that are small and of a kind which no
one would dispute. The jump from the initial statement to the
final one may seem large, but when broken down to a sequence of
small steps it does not appear so. The logic used in mathematics is
Keywords: Polya, formal proof, number patterns, algebra, pattern, sequence
At Right Angles | Vol. 2, No. 3, November 2013
37
actually no different from that used in ordinary
life (though it may seem different, especially when
expressed using symbols and formal mathematical
language); indeed, daily life is the source of all
logical methods. You could say, in fact, that much
of mathematical logic is plain and simple ‘kitchen
logic’!
It is believed by many that at the school level proof
is encountered mainly in the realm of geometry;
and that geometry is the only platform available
for teaching proof. Both these statements are false.
Proof lies at the heart of mathematics, in every
single branch. At the school level, one resource
that is heavily underutilized with regard to the
teaching of proof is Number Patterns and Algebra.
In this column we shall demonstrate many
principles of proof using themes from number
theory (which at this level is mainly applied
algebra). Of course, we shall consider themes from
geometry too.
It is equally a fallacy to imagine that proof can be
introduced only when students are in their upper
primary classes or in high school. Formal written
proof, yes; symbolic proof, yes; but informal and
clearly articulated, verbalized reasoning can and
should be introduced much earlier — indeed, in
the lower primary years. We shall elaborate on
this theme in subsequent columns.
An example from algebra
In the first ‘episode’ of this serial we study an
example from number theory:
Show that the square of any odd number leaves
remainder 1 when divided by 8.
We experiment with some numbers to get a sense
of the task: 12 = 0 × 8 + 1, 32 = 9 = 1 × 8 + 1, 52 = 25
= 3 × 8 + 1, 72 = 49 = 6 × 8 + 1, 92 = 81 = 10 × 8 +
1, 112 = 121 = 15 × 8 + 1, 132 = 169 = 21 × 8 + 1, .
. . . We see that the claim has worked for the odd
squares from 12 till 132. Is this enough evidence to
conclude that the pattern will always be true?
Not quite! As we said earlier, empirical evidence
is suggestive of the truth of a proposition — but
that’s all. In number theory there are numerous
instances of statements which fail despite the
38
At Right Angles | Vol. 2, No. 3, November 2013
evidence in their favour being very strong. A well
known example of this is Euler’s prime-generating
function n2 + n + 41, which yields prime values for
40 consecutive values of n (namely, n = 0, 1, 2, 3, . .
., 39; we get the primes 41, 43, 47, . . . , 1447, 1523,
1601), and just as we are beginning to be certain
that the expression will always yield a prime, the
formula disappoints us: the pattern breaks, with
n = 40 yielding a composite number. (It is easy to
check that n = 40 does yield a composite number,
for 402 + 40 + 41 is clearly a multiple of 41.
Indeed, it equals 412.)
So if we want actual proof then we have to
produce something that will stand up in the
‘mathematical court’ before the toughest lawyer,
who will be looking for ways to dash your
arguments to bits. Here are some approaches
which should satisfy such a lawyer.
First proof. What is an odd number? Clearly, one
that leaves remainder 1 when it is divided by 2.
This means that an odd number A is of the form
2 × an integer + 1, i.e., A = 2n + 1 where n is a
positive integer. Let us see what happens when
we square this expression:
A2 = (2n + 1)2 = 4n2 + 4n + 1.
We see readily that A2 is of the form 4 × (some
integer)+1. That is, A2 leaves remainder 1 when
divided by 4. While this comes close, it is not good
enough: we need division by 8, not by 4. What do
we do now?
Let’s look more closely. We see that
A2 = 4n (n + 1) + 1. If only we can show that
n (n + 1) is an even number, then our task will be
done, for the number 4n (n + 1) will then be twice
a multiple of 4, and therefore a multiple of 8.
But n (n + 1) is even; for, it is the product of two
consecutive numbers, of which one clearly must
be even. So our job is done!
Second proof. This approach may appear a bit
strange at first but is perfectly valid. The idea
comes from the fact that the problem has to do
with division by 8, so it seems natural to check if
there is some underlying pattern which repeats
each time n increases by 8. So we consider the
expression: (n + 8)2 − n2. We have:
(n + 8)2 − n2 = (n2 + 16n + 64) − n2 = 16n + 64 = 8
(2n + 8).
We see clearly that the last quantity is a multiple
of 8. So when n increases by 8, the remainder in the
division n2 ÷ 8 stays unchanged.
It follows that if the given statement is true for
the odd squares 12, 32, 52 and 72, then it will
necessarily be true for 92, 112, 132 and 152; and
therefore it will necessarily be true for 172, 192,
212 and 232; and so on, indefinitely. But the
statement is indeed true for 12, 32, 52 and 72, as is
easily checked. Therefore it is true for the square
of every odd number!
Remark. This proof can be hugely improved
once we notice that we do not need to consider
integers separated by a gap of 8. In fact, since we
are studying the squares only of odd numbers, a
gap of 2 is good enough! For, if we consider any
two consecutive odd numbers, say 2n − 1 and 2n +
1, the difference between their squares is
(2n + 1)2 − (2n − 1)2 = (2n − 1 + 2n + 1) × 2 = 4n ×
2 = 8n,
which is a multiple of 8. So if the hypothesis is
true for the first odd square (namely: 12), which it
clearly is, then it will be true for every subsequent
odd square. Hence proved!
Third proof. Just for variety we give a third proof.
It is based on the fact that the sum of the first n
odd numbers is n2. For example, 1 + 3 = 4 = 22 and
1 + 3 + 5 = 9 = 32. So to show that (2n − 1)2 is 1
more than a multiple of 8, we must show that the
sum of the first 2n − 1 odd numbers is 1 more than
a multiple of 8.
Now we observe the following simple pattern in
the sequence of odd numbers: the sums 3 + 5, 7
+ 9, 11 + 13, 15 + 17, . . . are all multiples of 8. It is
easy to see why this must be so; for, 3 + 5 = 8, and
in advancing from 3 + 5 to 7 + 9 we increase the
sum by 4 + 4 = 8. Likewise, in advancing from 7
+ 9 to 11 + 13 we increase the sum by 4 + 4 = 8.
As the sums increase by 8 each time, and we start
off at a multiple of 8, the sum will always be a
multiple of 8.
The statement now proves itself; for, in the sum
of the first 2n − 1 odd numbers, we can pair the
last two odd numbers, then the two odd numbers
just before that pair, and so on, down to {3, 5}.
The sum of each pair is a multiple of 8, and the
remaining number, 1, ensures that the sum is 1
more than amultiple of 8. The following depicts a
typical situation:
92 = 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17.
Closing remarks. We quote Professor Gila Hanna, from [1]:
The recognition that proofs can convey new mathematical techniques
effectively, and thus should be treated as important bearers of mathematical
knowledge, is a fertile point of view that mathematics educators seem to
have overlooked to a large extent. Adopting this approach to proof in the
classroom does not challenge in any way the accepted “Euclidean” definition
of a mathematical proof (as a finite sequence of formulae in a given system,
where each formula of the sequence is either an axiom of the system or is
derived from preceding formulae by rules of inference of the system), nor
does it challenge the teaching of proof as a Euclidean derivation. It is rather
an acknowledgement that the teaching of proof has the potential to further
students’ mathematical knowledge in other ways. It offers an opportunity
to make new connections between the process of proving and mathematical
techniques, and also gives us an additional reason for keeping proof in the
mathematics curriculum.
At Right Angles | Vol. 2, No. 3, November 2013
39
References
[1] Gila Hanna, Proof can teach you new methods, http://www.unige.ch/math/EnsMath/ Rome2008/WG1/Papers/
HANNA.pdf
[2] David Reid, Understanding proof and transforming teaching, http://www.pmena.org/2011/ presentations/
PMENA_2011_Reid.pdf
SHAILESH SHIRALI is Head of the Community Mathematics Centre in Rishi Valley School (AP) and Director
of Sahyadri School (KFI), Pune. He has been involved in math education and math olympiads since the
1980s. He is the author of many math books addressed to high school students, and serves as an editor
for the science magazine Resonance and for the magazine At Right Angles. He is engaged in many outreach projects in teacher education through the Community Mathematics Centre. He may be contacted
on [email protected].
A poem on the
prime number
theorem
The prime numbers are mysterious because they have the two `opposing'
properties: there are arbitrarily large gaps in between them and they satisfy no
simple formula, while simultaneously their distribution is regular in the sense
of the famous prime number theorem. This theorem can be informally stated as
saying that the probability of a number n being prime is 1/log(n). This can be
poetically worded as:
Numbers in their prime -for no reason or rhyme,
show up at a rhythm
with probability 1/logarithm.
If this is a law they knew,
they also break quite a few
but then, that is not a crime!
-- B Sury
40
At Right Angles | Vol. 2, No. 3, November 2013